Automatic volume control



Nov. 4, 1952 Filed MaICh 5, 1949 w. F. KANNENBl-:RG AUTOMATIC' VOLUMECONTROL 2 SHEETS-SHEET l /NVE/V'OR `f E KNNENBERG Patented Nov. 4, 1952UNITED STATES FATENT OFFICE AUTOMATIC VOLUME CONTROL Application March5, 1949, Serial No. 79,815

2 Claims. 1

This invention relates to an improved system for the automatic volumecontrol of a sound program reproduced in a noisy listening area. It 1sparticularly useful when it is desired to make the program volume at alltimes high enough to override the noise in the area as this noise variesin level. The system herein described is accordingly termed acontinuously adjusting system, thereby distinguishing it from theinterval adjusting system where the program volume is controlled by thenoise level in an interval preceding the rendition of a program.

The general object of the invention is to provide an improved system ofapparatus by means of which a sound program may be satisfactorilyreproduced despite the presence of disturbing noise.

Systems intended for the same object are known to the art, for example,one is disclosed by E. Stanko in United States Patent 2,338,551, Januaryll, 194e. In that disclosure, as in the present one, there is shown avariable gain amplifier in the program reproducing channel, the gain ofwhich is varied in accordance with the difference of two rectinedvoltages, one of which varies with the program level fed to aloudspeaker while the other varies with the output of a microphonepicking up the disturbing noise. The present system, however, contains alimiter which insures stability without curtalment, at i either end, ofthe frequency range of either program reproduction or noise microphoneresponse; this is accomplished by limiting the additional change in gainas the noise increases beyond a reasonably high level, up to which levelthe gain of the program channel rises uniformly with rise in noise.

Besides insuring stability, the limiter prevents the undesirableexcessive sound level, and perhaps overload of the amplifier supplyingthe loudspeaker, which otherwise might come about in the presence ofextreme bursts of noise.

It is therefore an object of the invention to provide an electricalsound system of which the sound output continuously varies in level withthe level of disturbing noise, but with limitation oi variation of soundoutput in the case of extreme variation in noise.

The invention will be fully understood from the following description ofa preferred embodiment thereof, read with reference to the accompanyingdrawings, in which:

Fig. l is a block schematic of the system;

Fig. 2 is a circuit diagram of a variable gain amplier suitable for usein the system;

Fig. 3 is a circuit diagram of the circuits cooperating in the sectionof Fig. `l designated by the numeral 5U;

Fig. 4 is the electrical resistance characteristic of the shunt portion5l' of the limiter 5l shown in Fig. 3;

Fig. 5 shows the relation between the rise in gain of the amplifier ofFig. 2 with increase in net input voltage supplied to the network ofFig. 4; and

Fig. 5 is an actual characteristic )obtained using the system of Fig. l.

In all figures, like elements are designated by like numerals.

Referring to Fig. l, 5 indicates generally a program source, symbolizedby disc sound record 6 driven by mechanism (not shown) and traversed byan electromechanical reproducer consisting of a stylus 'i operating togenerate in coil 8 a voltage representative of the sound recorded ondisc The output voltage of coil 8 is amplified by the amplifiers of theprogram channel lll, which includes preamplifier i2, variable gainamplifier i3, line amplifier I4 and power amplifier l5. The iinalamplified voltage supplies power to loudspeaker 20, installed as may bedesired in the listening area indicated by the dashed rectangle I.

Also, in the listening area is located microphone 3Q, whereby noise inthe area and also program sound the'rein is represented by an electricalquantity, voltage or current, amplified by the amplifiers of themicrophone channel 40. These include preamplifier l2 and line amplifier44.

It is here appropriate to anticipate part of the later description bypointing out that, in setting up the system of the invention, one soarranges the gain of the microphone channel that the power level at theoutput of line amplifier 44 shall be the same as the power level at theoutput of amplifier lil when there is no noise in the listening area andthe gain of the program channel has been set to provide a satisfactorilyhigh sound level in the area. Hereinafter the output terminals of theline amplifiers It and M will be referred to as bridging points.

Between the bridging points is connected a differential rectifier 59,the elements of which are described in detail in connection with Fig. 3.In the differential rectier, means is provided for connecting inopposition rectified voltages corresponding respectively to the outputof amplier lll and to that of amplier 44. These rectifiedvoltages differwhen noise appearsin'the 3). listening area and the difference voltageis supplied to increase the gain of variable gain amplifier iis andtherewith raise the sound level in the area to be satisfactorily abovethe noise level therein.

Referring now to Fig. 2, variable gain amplier l 5 of Fig. 1 includes amultigrid tube Vi, suitably a 1512, to control grid le of which isapplied via a conventional connection the alternating voltage betweenterminals li and it, which are the output terminals oi preamplifier l2of Fig. 1. Shielded grid i9 of tube VI is connected to the blade ofswitch S, which in position on is connected to terminal 28 of thecontrol bias circuit of Fig. 3 and in position ofi to ground. Grids leand i9 are interconnected by two resistors in series as indicated inFig. 2, the junction of these resistors being capacitatively connectedto cathode 2i of tube Vl. Anode voltage for tube Vi is supplied throughwell-known filtering means from a Z50-volt B battery. Anode 22 isconventionally connected to the control grid of a cathode follower tubeV2, preferable a 6J? connected as a triode, resistor 23 being theconnection between ground and cathode 2t of tube V2. The voltage across4500ohm resistor 23 is applied to conductors 25, 2i', leading to amplierit.

Between terminals ii' and i8, the latter being grounded, is connected a60G-ohm resistor 25. It is understood that a like resistor (not shown)may if desired be connected across terminals and 2i which are inputterminals of line amplier lit. Terminal 21 is grounded, and terminal 2tis connected by a capacitor in series with a resistor to cathode 2d.

In the absence of voltage from the control bias circuit at terminals 2Band 29, when switch S is closed on, the voltage gain of tubes Vi and V2together is adjusted, by known procedures, to be zero decibel. Apositive bias voltage applied to grid i9 increases correspondingly thegain of the circuit of Fig. 2 and therewith the power supplied toamplifier i5 and the sound level emitted by loudspeaker 2Q. Thedescribed operation of tube Vi is well known, being disclosed forexample, in United States Patent 2,245,652, June 17, 194:1, to J. E.Dickert.

It is understood that for the tubes Vi and V2 and others to bementioned, cathode heating power (not shown) is provided.

Control bias, corresponding to noise in the listening area, is derivedfrom the circuit of Fig. 3, which shows the elements included indifferential rectier 5B of Fig. 1.

Referring again to Fig. 1, diiferential rectier 50 includespreampliiiers 5i and 52, which re spectively amplify the output voltagesof line amplifiers lil and tl and supply these amplified voltages torectifiers 53 and 5d. In these rectiers the program voltage of channeliii and the program plus noise voltage in channel itil are rectified andthe resulting undirectional voltages are supplied to the respective timeconstant networks 55 and 5e. The voltage across the output of network 55is combined, in opposition, with that across the output of network 55 inlimiter 5i. From limiter 5i, a voltage representing the noise in thelistening area is supplied to control the gain of Variable gain amplieri3 of the program channel and therewith the sound level emitted byloudspeaker 20.

In Fig. 3 the numerals 5i and 52 are applied to designate generally thecircuits of the preamplifiers correspondingly designated in Fig. 1

4 within differential rectifier 50. Each of these preampliers includes a6SN7 tube, V3 and V5, respectively, each comprising two triodes to whichin push-pull are connected the program power (amplifier ifi) and themicrophone power (amplifler ed) at the respective bridging points.

Inasrnuch as amplifier 65 is terminated by the input impedance of poweramplifier i5 while amplier ed has no termination other than the muchhigher input impedance of preamplier 52, it is proper for circuitsymmetry to provide resistor ti across the input of preamplier 52 tomake the input impedance thereof equal that of power amplifier i 5.

Anode 53 of tube V3 is connected through a condenser and a resistance inseries to rectifier 53, which consists of a pair of Western Electric 34Avaristors, or equivalent, connected in parallel opposition. Theterminals of these varistors, remote from their common connection to theresistance, are connected respectively to the outer terminals ofcondensers ii-5Fl in series. To the junction ci these ccndensers anode59 is com nected through a condenser and a resistance in series,similarly to the connection of anode et to rectier 53. Each ofcondensers Si, t2 is shunted by a 2-megohm resistance, as shown, for thepurpose of providing definite leakage resistance. It is clear thatcondensers @i and t2 are charged, on alternate polarities of the programvoltage, to a like potential, constituting together a voltage doubler.

In like manner the two triodes ci tube V3 are supplied in push-pull withthe voltage from line amplifier #it of the microphone channel. Thisvoltage combines the program and noise as picked up in the listeningarea by microphone 30.

Of tube Vfl, anodes 53 and @It are connected, each through a condenserin series with a resistance, as are anodes and iis of tube V3, anode 53to rectifier 5ft like rectifier 53, anode iid to the junction ofcondensers Sie and iii shunted each by 2 megohms. The outer terminals ofthese condensers are connected to rectifier 54 as are the like terminalsof condensers @i and @E to rectifier 53.

As is indicated in the circuit of Fig. 3, ccndensers 6i and 52 arecharged in series with polarities as shown to a voltage representing theprogram level at the output of ampliiier iii, while the voltage to whichcondensers te and El are charged represents the program plus noise levelat the output of amplifier lit. The polarities ci condensers 6I, 2, 66and S1 (each 1000 microfarads) are also shown in Fig. 3, but it will benoted that points A and C are positive while points B and D are negativein potential. In order to subtract the program power from the programplus noise power, points B and D are connected through two resistors E53and es in series, each resistor being suitably of 60,000 ohmsresistance. Consequently, the voltage between points A and C is thedifference of the voltages A-B and C-D and so represents a dierentialvoltage corresponding to the noise alone. It is this differentialvoltage which is supplied to limiter 5i. Limiter 57 comprises a seriesportion and a shunt portion 5i'. The series portion is a linearresistance of 240,000 ohms composed of the four 60,000-ohm resistors 58,60, 73 and 76. The shunt portion is composed of resistive elements 10and il, a 240,000-ohm resistor and a varistor respectively, and250-microfarad condenser 12. The resistive parallel components alonedetermine the steady-state (resistance) characteristics of the shuntcircuit whereas both Aresistive and capacitive parallel elementsdetermine its transient behavior. The differential voltage (voltage A-Bless voltage C-D) is applied across the series1 and shunt portion of thelimiter in series. Only a fraction of this voltage therefore appearsacross the shunt portion. When the impedance of varistcr 'ii is largecompared to the resistance of resistor l0, the voltage ucr-oss resistorlo is one-half the differential voltage. When the impedance cf varistor'fi falls with increasing voltage the fraction of voltage appearingacross resistor "sii will become less than half that appearing betweenand C. This variable fraction is the useful output voltage of thelimiter accordingly appears across its output terminls 28 and 23, whichare connected, respectively, to grid Eil of tube Vi when switch S isclosed "o-n and to ground. Limiter 51 receives at input terminals asteady or .slowly varying direct-current voltage and passes on by way ofits output terminals a steady or slowly varying direct-current voltagewhich is never more than, and sometimes considerabiy less than, half theapplied input voltage namely, that between A C.

Resistors 68 and 59, in addition to contributing to the time constant ofthe series portion of limiter 5l, serve to prevent the harmful effectwhich in their absence would be produced by the grounding of terminal29.

Where important, and not elsewhere given, values of resistance andcapacity are shown numerically in Figs. 2 and 3 in thousands (K) ofohms, in megohms (M), and in microfarads. Gther and conventional circuitelements are not so identified.

Varistor li is constituted of three thalliumcopper oxide pellets inseries, the forward direction of which is from terminal 28 to terminal25. Terminal 28 being positive to ground, the voltage thereby applied togrid IS of tube VI increases correspondingly the gain of amplifier I3,Fig. 2.

Referring now to Fig. e, the resistance of the parallel connection ofresistor l@ and varistor II is shown to vary from 240,000 ohms to 10,000ohms as the value of the applied voltage rises from zero to 0.47 voltper pellet, the drop in resistance beginning at 0.15 volt per pelletapplied between the terminals of resistor le. Correspondingly, of thedifferential voltage between program and microphone channels at theoutputs of their respective line ampliiiers the whole is effective up toa certain value, after which a smaller and smaller change is made ingain of amplier I3 for a given increase in noise level. By suitablechoice of size and number of copper oxide rectifier pellets, the noiselevel at which the droop begins and the rate at which it proceeds may beadjusted as desired.

As a result, the gain of amplifier I3 increases in equal step with thenoise in the listening area up to a selected level of that noise, beyondwhich further increase in noise level brings about progressively lessand less change in gain. The sound level from loudspear varies directlywith vthe gain of amplifier I3, up to the point of overload of poweramplier I5; the particular varrangement of limiter 5l above set forthavoided, in a trial installation, such overload of the power amplifierand resulted in an increase of program sound level with noise increasewhich was `stable and within satisfactory limits as regards programquality and ratio of program sound level -to noise.

6 Fig. y5 shows a plot of the gainiof amplifier I3 as a function of thevoltage-applied across the limiting network 51. Volts as abscissae areplotted on a logarithmic scale, while amplifier gain in decibels isplotted on a like scale as ordinates. The' limiting region of ampliergain is shown in Fig. 5 as beginning at 1.0 volt input to the networkpreceding shunt portion 51'; this corresponds to 0.5 volt across portion51 itself.

Fig. 6 shows in three curves A, B and C, the operation actually obtainedwith a system as in Fig. 1, when background music from disc records wasplayed in a small restaurant where the noise level was capable of beingartifically varied.

For the sake of iilustration, consider the noise level at output ofamplifier 44 (in absence of program) to vary from -l0 to +25 dbm(decibels above one milliwatt). Assume the general program level to bedbm in the absence of noise at the output of amplifier i4, and that thislevel, lifted 10 decibels by power amplifier I5, gave from theloudspeaker a sound level 35 decibels .above the noise when the latterwas represented by -10 dbm at the output of amplier 44. Where there noprovision at all for raising the program level as the noise increased,this signal-to-noise ratio would fall from 35 decibels to zero decibelsat the highest noise level. If on the other hand, the program level wereto increase, decibel for decibel, with increase of noise, when thelatter reached +25 dbm the loudspeaker power would rise from +17.5 dbm(l0 decibels gain in amplier I5 added to +7.5 dbm at output of amplifier14) to +525 dbm with probable overload of the power ampliier andexcessively loud sound if a sudden break came in the noise. Curve Ashows the gain change in amplier I3 as the noise level at the bridgingpoint of the microphone channel rises from -10 to +25 dbm. Curve B showsthe corresponding rise in loudspeaker power above the +l7.5 dbm assumedat the noise level -10 dbm. Curve C represents the decrease insignal-noise ratio, from 35 decibels assumed for a noise level of -10dbm to 13.6 decibels at a noise levelof +25 dbm; without the automaticgain control of the program channel, the program music would have beenlost in the noise.

Curve A is the locus of points of stable equilibrium. At any point onthis curve the system is in static equilibrium, so that provided nochange in noise or program occurs, no change in controlled amplifiergain will occur either. For a system to have dynamic stability, however,the satisfaction of steady-state requirements is a necessary but not asuiiicient condition. To be dynamically stable as well, the system mustin addition possess a transient behavior which tends to contract ratherthan expand system gain.

Referring again to Fig'. 3, the reason for the stability of thedescribed system (meaning thereby the tendency for the bias controlvoltage to return to an earlier value as the noise level itself soreturns) is seen to lie in the fact that three suitably proportionedtime constant circuits are provided. These are: first, the two1,000-rnicrofarad condensers 5l and 52, each in series with a 7,500- ohmresistor and shunted by 2 megohms, constituting the program channel RCnetwork 55 of Fig. 1; second, the similar condenserresistance circuitconstituting vthe microphone channel RC network 5t of l; third, thelimiter circuit 5l.

Suppose the condensers in each channel to be charged due to programrpower alone, `directly 4'received from amplier 5I by network 55, andlreceived by acoustic pick-up from the loudspeaker by network 56. Thecharging circuit of each of these condensers has a nominal time constantof '7.5 seconds (7,500 ohms 1000 microfarads). Since charging isdiscontinuous the effective time constant is much larger than this.Charging current will flow only when a voltage peak in the conductingdirection occurs, which already cuts in half the charging time.Furthermore charging current will flow only during that portion of aproperly poled peak in which the actually developed voltage exceeds thedirect-current voltage to which the condenser has already been charged;this further reduces opportunity for charging current to iiow, so thatthe charging time is halved again. Since the nature of the drivingvoltage obtained from either channel is generally not sinusoidal, butconsists of random alternately poled peaks oi' varying magnitude andspacing depending upon the nature of the music (over the programchannel) and upon the nature both of the music and of the noise (overthe microphone channel), charging time is cut further. Assume forexample that the cut in charging time assignable to this effect is toonly 10 per cent of the charging time reduced as above to 25 per cent.At 100 per cent charging time (as would occur with properly poledconstant amplitude D. C.) the v effective time constant would be 7.5seconds as calculated, but at 2.5 per cent the eifective time constantwill be 7.5 divided by 0.025 or 300 seconds. Thus, on the type ofdiscontinuous signal likely to be encountered, the condensers willcharge in roughly 300 seconds, or 5 minutes, to the same extent as anapplication of a continuous and constant voltage of the same value foronly 7.5 seconds would produce.

The discharge of these separate but similar circuits occurs through2-rnegohm resistors shunting each of condensers 6i, 62, 00 and 'i. Sincein this case the discharge occurs on a continuous basis, theeiiectiveness of discharge is 100 per cent, making the effectivedischarge time constant 2000 seconds or 331/3 minutes (2 megohms 1,000microfarads) Since rectiers 53 and 5ft and networks 55 and 50 of Fig. lare identical, condensers i, 62 and 56, 6l will respond equally rapidlyand in perfect step with each other to variations in average programpower, so that no difference voltage appears between points A and C andthe gain of amplifier 13 remains at Zero decibels as initially adjusted.On cessation of the program, the l000-microfarad condensers dissipatetheir charge at equal rate (at the 33-minute time constant rate) so thatthe gain of amplier i3 remains at zero decibel Ysince during dischargeno difference voltage will appear between points A and C.

ln the presence of noise, a difference voltage appears between points Aand C. At low values one-half of this voltage is passed on by thelimiter; at high values the transmitted fraction is less than half. Thusthe difference voltage between A and C is the source from which the RCcircuit l" comprising condenser l2 and resistor l0 charges through thefour 60,000-ohrn resistors t8, 69, i3 and ld. Thus, at cessation ofnoise the full difference voltage between A and C across four1,000-microfarad condensers in series, minus the voltage across shuntportion 5l (approximately half that between A and C) attempts todischarge through the four 60,000-ohm resistors. On this basis, thedischarge time constant of the circuit tied to A and C is 1000/4microfarads 240,000

ohms, or 60 seconds. But circuit 5l', in complete absence of energysupplied from A and C, has a discharge path through resistor l0 which isof 240,000 ohms resistance. At higher voltages varistor li makes thedischarge path lower in resistance. Hence maximum discharge timeconstant of this limiter circuit RC network acting alone is 250 10-6240,000, or 60 seconds. Both circuits having identical time constants,the discharge time constant of their combination is twice that, orseconds. This is the case for voltages across the limiter shunt path 5l'of 0.5 volt or less. At higher voltages than this the impedance ofvaristor li begins t0 fall rapidly to values approaching 10,000 ohms.Accordingly the time constant of the circuit 5l approaches 2.5 seconds.Therefore at higher voltages the overall discharge time constantapproaches that of the voltage difference source, A and C, or 60seconds. Accordingly, a higher limiter output voltage will tend to bereduced at a rate almost twice as fast as one of 0.5 volt or less.

Thus, since the effective charging time constant controlling voltagebuild-up on the condenser banks is of the order of nve minutes, whilethe effective normal discharge time constant applying to diierentialvoltages is of the order of two minutes, the direct-current output ofthe limiter will tend to follow slowly changing ambient noise quitefaithfully except for the steadystate limiting effect previouslydescribed in connection with Fig. 6, and for the tendency to restorefaster from a high output voltage. Transient behavior as a result ofnoise bursts of prolonged duration brings about operation as follows:

The iirst of the prolonged noise bursts starts driving up the voltageacross the microphone channel condensers S0 and 0l, which increases thevoltage dierential, the controlled amplier gain, and the program channelvoltage on condensers iii and 62. But the sound level in the listeningarea due to program is also raised so that as end result both thesecondenser voltages and the voltage diierential A-C are appreciablyhigher than normal. The effect of the higher condenser voltages is toincrease the effective charging time constants of condensers 0I, 62, 66and 6l' since a given voltage peak applied will cause less chargingcurrent to now than before. As previously pointed out, the excessivediierential voltage will tend to restore rapidly at the rst opportunitybetween noise blasts, thereby bringing down the elevated program soundlevel. Since the voltages of condensers 5i, 02, 60 and 5l will dissipaterather slowly, the temporarily reduced sensitivity of the chargingcircuit will tend to restrain the adverse effects of succeeding noisebursts until previous conditions are completely reestablished. Theextent of this secondary effect is controllable by adjustment of thedischarge time constant of the condenser banks (change of the Z-inegohmresistors to some other value).

Thus the advantages resulting from the provision of limiter 57i are:iirst, at normal noise levels and normal rates of variation in generalnoise level, the variation of loudspeaker level is unobtrusive; second,at suddenly occurring prolonged noise bursts the program level increaseoccurs initially at normal rate, but subsequent bursts are less capableof driving up loudspeaker level; third, the higher the previous noiselevel, the more rapidly will the reduction in sound output occur withcessation of the noise or the sudden reduction in the level thereof. Theresult is in all cases to make the program rendition follow averagenoise level without disconcerting upward or downward spurts, withoutperiods of excessive signal-to-noise ratio, and with continuousstability.

What is claimed is:

1. A sound reproducing system adapted to reproduce sound in a noisylistening area at a sound level appropriately related to the noise levelin the area comprising, in combination, an electrical sound reproducingchannel of variable gain, an electrical channel including a microphoneresponsive to the reproduced sound and the noise and means foramplifying the response of the microphone, means for deriving from thefirst-named channel a rst unidirectional voltage of magnitudecorresponding to the sound level, means for deriving from thesecond-named channel a second unidirectional voltage of magnitudecorresponding to the amplified response of the microphone, means forderiving from the first and second voltages a differential voltage,means for progressively limiting a fraction of the differential voltagewith progressive increase of the diierential voltage and means forapplying the limited fractional voltage to vary the gain of thefirst-named channel.

2 A sound reproducing system as in claim 1 wherein each of theunidirectional voltages is impressed on a resistance-capacity circuithaving a charging time constant nominally of the order of seconds and adischarging time constant of the order of minutes, and wherein thelimiting means comprises a resistance-capacity circuit having a timeconstant decreasing from the order of minutes to the order of secondswith increase in value of the diierential voltage.

WALTER F. KANNEN'BERG.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 1,356,403 Pridham Oct. 19, 19202,338,551 Stanlio Jan. 4, 1944 2,382,343 Baumgartner Aug. 14, 19452,392,218 Anderson Jan. 1, 1946 2,420,933 Crawford et al. May 20, 19472,462,532 Morris Feb. 22, 1949 2,501,327 Good Mar. 21, 1950

